docs: use the `escape_svg` function.

site/docs/4.3/assets/scss/_nav.scss

@@ -67,7 +67,7 @@
     font-weight: 600;
     color: $gray-900;
     background-color: transparent;
-    background-image: url("data:image/svg+xml,%3csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 8 8'%3e%3cpath fill='%23292b2c' d='M2.3 6.73L.6 4.53c-.4-1.04.46-1.4 1.1-.8l1.1 1.4 3.4-3.8c.6-.63 1.6-.27 1.2.7l-4 4.6c-.43.5-.8.4-1.1.1z'/%3e%3c/svg%3e");
+    background-image: escape-svg($dropdown-active-icon);
     background-repeat: no-repeat;
     background-position: .4rem .6rem;
     background-size: .75rem .75rem;

site/docs/4.3/assets/scss/_variables.scss

@@ -7,6 +7,7 @@ $bd-download:      #ffe484 !default;
 $bd-info:          #5bc0de !default;
 $bd-warning:       #f0ad4e !default;
 $bd-danger:        #d9534f !default;
+$dropdown-active-icon: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 8 8'><path fill='#292b2c' d='M2.3 6.73L.6 4.53c-.4-1.04.46-1.4 1.1-.8l1.1 1.4 3.4-3.8c.6-.63 1.6-.27 1.2.7l-4 4.6c-.43.5-.8.4-1.1.1z'/></svg>");
 
 // Enable responsive font sizes for font sizes defined in the docs
 // The weird if test is made as a workaround to prevent a false fusv error.